Add spline detection function

[jcarlis3]

Create a smooth.like option for F.dfunc.estim that fits a spline to distances.

Some notes from discussing this option:

smooth.like <- function(b, dist, w.lo, w.hi, series, expansion, scale){
     L1 <- density(dist, width=b, n=1024)
     plot(L$x, L$y)
     L <- approx(L1, xout=dist)$y
     L
}

MASS::width.SJ(dist)
bs(dist)